As an energy conversion technology utilizing thermoelectric conversion, a thermoelectric power generation technology and a Peltier cooling technology have been known. The thermoelectric power generation technology is a technology that utilizes conversion from thermal energy to electric energy through the Seebeck effect, and the technology is attracting increasing attention particularly as an energy saving technology capable of recovering, as electric energy, unused waste heat energy formed from the fossil fuel resources or the like used in buildings, factories, and the like. The Peltier cooling technology is a technology that utilizes conversion from electric energy to thermal energy through the Peltier effect in contrast to the thermoelectric power generation, and the technology is being used in a wine refrigerator, a small portable refrigerator, cooling for a CPU used in a computer or the like, and a component or device that requires precise temperature control, such as temperature control of a semiconductor laser oscillator for optical communication. However, the technologies have a low thermoelectric conversion efficiency, and the practical applications thereof are still limited to the aforementioned fields.
In recent years, while it is common that an electronic device has mounted thereon a semiconductor element relating to the operation and control thereof, the semiconductor element is heated to a high temperature and becomes by itself a heat generator radiating a large amount of heat, due to the reduction in size thereof through miniaturization, the enhancement of the performance thereof, and the like. Under the circumstances, a cooling device that efficiently absorbs heat from the semiconductor element is demanded to be reduced in size.
One measure therefor is electronic cooling utilizing the Peltier cooling technology, but an ordinary Peltier element is a thermoelectric element that uses a sintered material of a thermoelectric material, which results in limitation in the aspects of the mechanical strength in size reduction, the installation mode on a surface of a heat generator (for example, mounting on a flexural area), the precision, and the like, and therefore it is demanded to provide a Peltier element in a sheet form including the formation of a thin film of a thermoelectric material by a coating process, such as printing, and the flexibility of the element.
The thermoelectric conversion utilizes the physical phenomena that are peculiar to materials, such as the Seebeck effect and the Peltier effect, as described above. However, for enhancing the efficiency of thermoelectric conversion, it is necessary to develop a thermoelectric conversion material having a high performance. The thermoelectric conversion efficiency can be evaluated by the figure of merit Z (Z=σS2/λ=σΠ2/λT2). Herein, S represents the Seebeck coefficient, Π represents the Peltier coefficient, σ represents the electroconductivity (which is the inverse of the resistivity), λ represents the thermal conductivity, and T represents the absolute temperature of the junction part. In view of this, for enhancing the efficiency of cooling, it is important to find a thermoelectric conversion material that has a large Seebeck coefficient S relating to power generation, a large Peltier coefficient Π relating to cooling (the Peltier coefficient and the Seebeck coefficient are in proportional relationship assuming that T is constant), a large electroconductivity σ, and a small thermal conductivity λ.
Under the circumstances, PTL 1 describes a method for manufacturing a thermoelectric conversion element aiming the enhancement of the power generation efficiency and the efficient production thereof, in which solutions having insulating property providing materials of p-type and n-type organic semiconductor elements are coated or printed on a support, and then dried. NPL 1 describes investigations of production of a thin film thermoelectric conversion element, in which a composition containing bismuth telluride dispersed in an epoxy resin as a thermoelectric conversion material is formed into a film by coating. PTL 2 describes investigations of a thermoelectric material, in which an organic thermoelectric material, such as a polythiophene or a derivative thereof, and an inorganic thermoelectric material, are integrated as a dispersed state.